1. Field of the Invention
For the past forty years the normal procedure for beneficiating Florida phosphate rock from associated gangue minerals has included a double flotation (or Crago flotation) step for that portion of the mineral feed in the size range of approximately 0.1-1 mm, which portion generally contains an appreciable fraction of the phosphate values. In this flotation process, which removes the major contaminant (quartz sand), the deslimed feed rock is first subjected to flotation using a mixture of fatty acids and fuel oil as collector. The resultant overflow retains most of the phosphate mineral and some entrained quartz. After deoiling, this rougher concentrate is subjected to a reverse flotation step using cationic collectors such as primary amines, which floats off most of the remaining quartz and retains a highgrade phosphate concentrate in the underflow. Since its introduction, this flotation procedure, as described by A. Crago (U.S. Pat. No. 2,293,640), has been successfully employed for siliceous central Florida phosphate deposits with little subsequent process modification.
However, the ore of the Florida Bone Valley Formation most suited to this beneficiation procedure is rapidly being depleted. Lower quality ore from the southern extension of the central Florida phosphate field is now of necessity becoming commercially exploited. This lower quality ore from the Hawthorn Formation is somewhat mineralogically different from that of the Bone Valley Formation. The Hawthorn ore is generally less weathered or altered and usually contains appreciable quantities of alkaline earth metal carbonate minerals such as dolomite. Surface chemical properties of carbonate minerals such as dolomite, calcite, or dolomitic limestone are often very similar to the surface chemical properties of the predominant phosphate mineral in the ore, a sedimentary marine carbonate-apatite known as francolite. For instance, the generally recognized similarity in response of calcite, dolomite, and francolite minerals to fatty-acid flotation collectors is believed largely due to specific adsorption and bonding of the fatty acid to the mineral surface by salt-like complexing of the fatty-acid carboxyl moiety with the surface calcium ions common to all these minerals. Because of these surface similarities, it is difficult to separate carbonates from the phosphate minerals by physical beneficiation methods such as flotation, which are dependent for their success on exploiting dissimilarities in the surface properties of the minerals to be separated. Hence, the Crago double-float process has unfortunately been ineffectual in separating carbonate gangue from the phosphate values.
The presence of carbonate in the phosphate concentrate is undesirable; it not only acts as a P.sub.2 O.sub.5 diluent, but also is detrimental in subsequent chemical processing of the rock. In phosphoric acid or superphosphate manufacture, for example, the presence of carbonates consumes additional sulfuric acid in the acidulation steps without providing additional fertilizer values. Carbonate also exacerbates foam formation in the reactor vessels thereby reducing their effective production capacity. The presence of appreciable MgO in the phosphate concentrate (e.g., MgO&gt;1%), as derived from dolomite or dolomitic limestone, is particularly objectionable in the manufacture of wet-process phosphoric acid (hereinafter referred to for the sake of convenience simply as WPA) since a significant MgO content in the resulting product WPA causes deposits a sludges and scale during and after processing of the rock concentrate to phosphoric acid. Because of this inability of the Crago double-float process to separate gangue minerals other than silica, for example, carbonates such as dolomite and calcite, from the phosphate concentrate, it is thus readily apparent that as the supply of high quality phosphate ore is being depleted there is a most pressing need for a flotation process suitable for Florida and similar phosphate rocks whereby both the siliceous and carbonate impurities therein can be separated effectively from the phosphate values.
2. Description of the Prior Art
The prior art teaches that several attempts have been made to develop processes for separating carbonate minerals such as dolomite from the phosphate mineral contained in Florida-like phosphatic ores where silica is the principal impurity. In all the prior art where particle size of the rock feed permits effective separation by flotation, siliceous materials, such as quartz sand, and alkaline earth metal carbonates, such as dolomite, are removed in separate flotation steps. The silica is removed using one or both stages of the Crago double-float method. The carbonate is subsequently or sometimes previously removed from the phosphate values in another separate and distinct flotation stage, often requiring a different collector reagent and, in all instances, use of a flotation depressant for either phosphate or carbonate.
For example, in U.S. Pat. No. 4,287,053 (assigned to the assignee of the instant invention), J. R. Lehr et al teach that dolomite was removed from the phosphate mineral by floating off the dolomite using fatty-acid collectors while depressing the phosphate mineral flotation by addition or organic phosphonic acids. In another such related teaching, R. E. Snow in U.S. Pat. No. 4,364,824 discloses that dolomite was floated off using sulfonated fatty-acid collectors while the phosphate mineral flotation was suppressed by addition of depressants such as sodium tripolyphosphate. Conversely, in U.S. Pat. No. 4,144,969 R. E. Snow teaches that the phosphate mineral was preferentially floated using primary amine collectors, while dolomite was removed in the underflow using fluoride as a depressant.
All such beneficiation schemes supra are complex, with the severe disadvantage that maintenance of two or usually three separate and distinct flotation circuits are necessary to ensure removal of both siliceous and dolomitic impurities. An attendant problem exists in that each flotation stage generally requires a separate conditioning step and often an additional processing step to remove reagents used in the previous flotation stage from the mineral surfaces prior to the next flotation stage. None of these processes have proven entirely successful and, as yet, no completely satisfactory beneficiation scheme exists for dolomitic phosphate ores of the Florida type.
Thus it is apparent that it is becoming increasingly desirable that there be developed or devised an improved method of beneficiating these ores, preferably by a technically less complex and more economical process requiring fewer flotation stages to remove both carbonate, particularly dolomite, and siliceous impurities from the phosphate ore.
Our new and unexpected discovery comprising the instant invention provides such a process wherein the surface properties of the ore are selectively and substantially modified in a novel manner by selectively coating the phosphate mineral surfaces with a molecular layer of metal sulfide. The procedure of the instant invention renders the phosphate mineral responsive to sulfide flotation collector reagents permitting subsequent separation of the phosphate mineral directly from both of the siliceous and alkaline earth metal carbonate impurities simultaneously in but a single flotation step.
In addition to decreasing the number of flotation circuits required by previously proposed schemes for separating the phosphate values from siliceous and carbonate gangue, our new and novel invention has the further distinct advantage of eliminating the use of reagents derived from tall oil in the flotation circuit. Tall oil is the main source of fatty acids required in the Crago flotation process and modifications thereof. However, the increasing scarcity, variable composition, and increasing cost of tall oil in recent years has made the use of alternatives therefore as flotation reagents in phosphate beneficiation increasingly attractive.